1. Field of the Invention
This invention relates to a microwave energy waveguide horn antenna having a tapered horn portion and straight waveguide portion which has walls formed of folded sheet metal.
2. Description of the Prior Art
Waveguide horn antennas are often used in transmission systems in which microwave energy is transmitted and received. Many of these transmission systems are of the stationary type in which the size and cost are not major problems. In portable transmission systems, such as hand-held television cameras, the size and cost of certain components of such systems including the waveguide horn antennas have become critically important.
Known waveguide horn antennas are relatively expensive to fabricate. A typical prior art waveguide horn antenna is shown in FIG. 1. The waveguide horn antenna, generally designated as 10, is comprised of a waveguide portion 12 having a flange 14 and a horn portion 16 having a flange 18. Flanges 14 and 18 are joined together as by screws 20 to form the horn antenna 10.
Typically, waveguide portion 12 is machined to a rectangular cross-section. However portion 12 can be formed relatively inexpensively in large quantities by extrusion techniques because of its simple, uniform geometric shape. Also, as disclosed in U.S. Pat. No. 3,925,883 isued on Dec. 16, 1975, instead of being machined or extruded, three walls of the waveguide portion 12 may be formed by bending a metallic sheet into a rectangular channel and a second metallic sheet comprising the fourth wall sealed to the channel to form the tubular rectangular waveguide. Other patents which describe methods of forming tubular structures from sheet metal are U.S. Pat. No. 2,115,441 issued on Apr. 26, 1938 and U.S. Pat. No. 2,996,790 issued on Aug. 22, 1971, neither of which discloses the present invention.
The horn portion 16, being of a tapered rectangular cross-section, is more difficult to fabricate because of its irregular form. Horn portion 16 is usually fabricated by machining or by a well-known electroforming process which consists of plating a material, such as copper, to a given thickness on a mandrel formed of another material, such as aluminum. The mandrel is shaped to provide the inner dimensions of the horn portion desired. The mandrel is subsequently etched out by a chemical solution which does not react with the plated material leaving thereby the horn structure but destroying the mandrel. This process of electroforming, which requires the use of a separate mandrel for each horn portion, is so costly as to be prohibitive for fabrication of horn antennas in a portable microwave transmission system.